This incredible display isn't some a futuristic domed city in an alien icy landscape or an artist's concept of magnetic fields, but a very real optical phenomena here on Earth. Here's the science behind these spectacular ice halos photographed this week in New Mexico.
These ice halos were photographed by Joshua Thomas on January 9, 2015 in Red River, New Mexico, identified by Les Crowley, then annotated by the National Weather Service station in La Crosse, Wisconsin. So, what's going on? A lot.
Base image credit Joshua Thomas. Annotated image credit Joshua Thomas/National Weather Service.
Ice halos happen when tiny crystals of ice are suspended in the sky. The crystals can be high up in cirrus clouds, or closer to the ground as diamond dust or ice fog. Like raindrops scatter light into rainbows, the crystals of ice can reflect and refract light, acting as mirrors or prisms depending on the shape of the crystal and the incident angle of the light. While the lower down ice only happens in cold climates, circus clouds are so high they're freezing cold any time, anywhere in the world, so even people in the tropics mid-summer have a chance of seeing some of these phenomena.
Explaining the optics of these phenomena involves a lot of discussing angular distances. To get a sense of what that means, here's a rough conversion between common distances and your body. Hold your hand out at full arm's length, fingers spread. Your thumb is approximately 0.5°: that's the size of a full moon or the sun (be kind to your eyes and test it by trying to cover the moon with your thumb, not the sun!). The distance from the tip of your thumb and your pinky is about 22°, a key distance when talking about ice halos. Now ball your hand into a fist: it's probably 10 to 15°. To figure out which, you're going to use both hands, and stack your fists one on top of another from straight sideways to directly overhead. Count how many fists that took, divide 90° by that number, and you've got your fist-size. (Not relevant today, but thanks to the Earth's rotation, the sun and stars move at 15° an hour relative to us, so you can do things like use your newly-calibrated fists to measure how long it will be until sunset!)
Working from the annotations, here's the various atmospheric phenomena happening from the common to downright rare:
One of the two most common ice halos, the 22° arc forms a circle around a light source that subtends a full 22°. That's about the distance covered by your fully outstretched hand, thumb on the sun to pinky finger on the halo. Sometimes only a part of the arc is visible, other times its the full circle. The inside of the hole is a crisply defined edge, sometimes lined in red, while the outer edge is a diffuse halo of white. A similar but smaller arc formed by water droplets is a corona.
A complete 22° arc circle in a veil of cirrostratus clouds. Image credit: NOAA
It is formed by light deflecting through the hexagonal face of any ice crystals in thin cirrus clouds or icy fog. No light is deflected less than 22° *, setting the minimum diameter of the arc and the hole in the center of the halo. Most light is deflected to approximately 22°, creating a bright inner edge, but some light is deflected up to 50°, blurring the outer extent of the halo.
* This is a tiny lie: red light is deflected 21.7º which can result in a red inner lining just a whisper inside 22°.
The other of the two most common ice halos, sun dogs also go by the name parhelion or mock suns. They're a pair of small, bright echoes 22° or further away to the right and left of the sun. They can be dim or blindingly bright (brightest when the sun is near the horizon), and tinted a variety of colours. Extremely well-developed sun dogs can have long white tails pointing away from the sun. They are frequently accompanied by a circumzenithal arc* if the sun is low. Despite the name, sun dogs can also happen around the moon.
* Circumzenithal arcs are an upside-down rainbow seen only when the sun near the horizon.
A matched pair of sundogs seen from Antarctica, South Pole Station in January 1979. Image credit: John Bortniak/NOAA
They are formed by light deflecting through thin ice plates in either high cirrus clouds or ground-level diamond dust. The hexagonal ice plates drift down nearly horizontally, so we see the light bent through the thin sides or deflected through multiple crystals. Like with the 22° arc, the minimum angle of deflection is 22°, so the light is focused into sun dogs at least that far from the sun. If the sun is higher in the sky, the light is internally reflected first, increasing the angle of deflection and setting the sun dogs farther from the sun.
The appearance of the tangent arc depends on the sun's position above the horizon, but they always stay in contact with the 22° halo at the arc vertex. The upper tangent arc and the lower tangent arc are effectively mirror images of each other. The lower tangent arc is only visible if the sun is at least 22° above the horizon.
Tangent arc kissing a 22° halo with a sundog to the side in seen from Saalbach, Austria. Image credit: Thomas Dossler
Starting as narrow V-shaped wings tangent to the top and bottom of the 22° halo at sunrise, the wings open broader and broader until the sun hits 29º above the horizon. At this critical juncture, the arcs open so much that the top and bottom tangents encircle the sun completely, forming a circumscribed halo, a brightly-coloured oval around the sun. Once the sun starts setting and drops below 29º, the halo splits into a pair of arcs* again, wings gradually flapping into tighter arcs..
* In the top photograph, the sun is low enough that only the upper tangent arc is visible and the lower arc is lost below the horizon.
The tangent arc is formed by light deflecting through side faces of a long hexagonal columns of ice drifting so the column is nearly horizontal. As before, the minimum angle of deflection is 22°, so light can bends through the crystals to produce wings starting 22° from the sun and arcing outwards. The prism effect can be strong, with red bending the least so colouring the base of the wings, extending out through the rainbow to green, blue or purple wingtips.
Sun pillars are vertical shafts of light above or below the sun and roughly the same diameter (0.5°). The upper pillar is typically brighter and easier to see as the sun is closer to the horizon, and can even be visible after sunset tracing out the path of the sun below the horizon (northward in the northern hemisphere, southward in the southern hemisphere). The lower pillar is brighter as the sun is higher above the horizon. They take on whatever colour the sun is, from pale yellow to the rich oranges, reds, and purples of sunset. The pillars are usually 5° to 10° tall, but can be a massive 30° in freezing fog. Pillars can also form around the moon or even a brightly-twinkling Venus.
Sun pillar photographed in San Francisco, California. Image credit: Brocken Inaglory
The pillars form by light reflected from the large, flat surfaces of ice plates wobbling as they fall. Because all it takes is light coming through a large, roughly flat surface, this type of halo happens even when the ice crystals are wonky, deformed, or otherwise imperfect. The light reflecting through the lower surface produces the upper pillar, and light reflecting through the upper surface produces the lower pillar. The greater the tilt of the crystal, the taller the pillar.
Variations on sun pillars can be formed by variations on either the shape or orientation of the crystals. If columnar crystals are falling, the pillar can be topped by tangent arcs. If plates are falling at an angle known as the Lowitz orientation, they can produce pillars simultaneous to a Lowitz arc in the position of a 22° halo.
A beautiful array of halos and arcs photographed at Marshall Space Flight Center in Huntsville, Alabama on October 30, 2012: a 22° halo, a parahelic circle connecting a pair of sun dogs, an upper tangent arc, an upper Lowitz arc, a helic arc, and a Parry supralateral arc. Image credit: David Hathaway/NASA/MSFC
From here we move into the more rare phenomena that require perfect crystals, exact orientations, or are so inherently faint that they're easily washed out by anything but the best conditions.
Infralateral arcs are bright, large arcs also known as lower lateral tangent arcs. Like tangent arcs, their appearance changes depending on the sun's distance above the horizon. When the sun is low, the arc is small, narrow, and tight to the lower sides of the 46° arc (a halos similar to the 22° halo, but bigger: a raidus of two full handspans). As the sun is farther from the horizon, the tangent point slips down. Once the sun is reaches 68°, the two arcs unite into a single arc clinging vertically to the 46° arc, slipping apart so the vertex is no longer tangent as the sun rises even higher. Infralateral can be bright and colourful.
Infralateral arcs are formed when light passes through the base of long hexagonal colums and exits out a side face.
The supralateral arc starts as a pair of arcs tangent to the sides of a 46° arc as the sun is close to the horizon, extending over the top of the halo until meeting as a single arc tangent to the circumzenithal arc and more than 46° from the sun as the sun rises. It is always above any parhelic circle, and disappears completely if the sun is more than 32.3° from the horizon.
From top to bottom, a colourful circumzenithal arc, a faint supralateral arc, an upper suncave Parry arc, a bright upper tangent arc, and 22° halo photographed in Salem, Massachusetts on October 27, 2012. Image credit: Joseph Thiebes
The arc is often mistaken for the massive 46° arcs, the larger cousin of 22° arcs. Ways to tell them apart include:
- Supralateral arcs change shape with the sun's changing altitude; a 46° arc maintains the same circular shape at all times (although only isolated arc fragments may be visible).
- The supralateral arc is a faint arc tangent to the circumzenithal arc; a 46° arc is only tangent while the sun is between 15° and 27° above the horizon and is otherwise separated from the arc.
- Supralateral arcs frequently accompanied by a strong tangent arc and weak 22° halo; a 46° arc is more likely accompanied by a weak tangent arc and strong 22° halo.
- Supralateral and infralateral arcs form cusps when they cross a parhelic circle; a 46° arc crosses without disruption.
- If the sun is at least 15° above the horizon, supralateral arcs are brightest at the top; a
46° arc is uniform brightness throughout its circumference. (This distinction can be obscured by clouds.).
- If the sun is more than 32° above the horizon, supralateral arcs do not exist; a
46° arc do exist.
- Supralateral arcs typically have brighter colours and more intense blues and greens; a 46° arc is more muted in colours and more frequently lacking blues or greens.
Like infralateral arcs, supralateral arcs are formed when light passes through the base of long hexagonal columns and exits out a side face. If the crystal tilt is low so the columns are nearly horizontal, the arcs are clear and distinct. If the crystals are more tilted, the arcs become more and more difficult to distinguish from 46° arcs.
Parry arcs are a subset of tangent arcs produced when the columnar crystals have a very distinct orientation. They mimic the change in shape of tangent arcs, but climb farther away from the sun as the sun moves farther from the horizon so that they start at a near-overlap with the tangent arc and gradually claw their way higher in the sky for upper arcs, or closer to the horizon for lower arcs.
Using a human to block out the sun makes it easier to see the halos: a 22° circle, a pair of sun dogs, a parhelic circle, an upper tangent arc, and a Parry arc. Photographed in December 1980 at the South Pole Station. Image credit: Lt. Cindy McFee/NOAA
Parry arcs happen when light passes through hexagonal columns that are in the Parry orientation: crystals falling so not only is the long axis horizontal to the ground, but the hexagon is oriented so flat faces are also parallel to the ground. (The Parry orientation allows one degree of freedom for movement: the ice column can spin sideways like a top, but is not toppling end-over-end or rolling like a log.) When the light passes through the side faces, the deflection will produce upper and lower Parry arcs that are either suncave or sunvex * curvature.
* "Suncave" means the arc is concave with respect to the sun, curving around it to form a bright cave; "convex" means the arc is convex with respect to the sun, curving away from it.
Helic arcs loop above the sun that are always white, not coloured, and usually quite faint. When only fragments are visible, they look almost like they're rays originating at the sun splaying outwards (although certainly not to the dramatic extent of crepuscular rays).
The loop is largest when the sun is closest to the horizon, vertex shrinking as it rises as the limbs spread farther east and west away from the sun. The loop stretches all the way to zenith when the sun is a scant 10° above the horizon. When the sun reaches 30°, the loop fits neatly within the circumzenithal arc; by 50°, the loop vertex is within the Parry arc. By the time the sun is a full 65° above the horizon, the limbs have stretched around the sky to intersect opposite the sun on the distant horizon.
Like the Parry arcs, helic arcs only happen when columnar ice crystals are in the Parry orientation. These arcs form when light externally reflects off the lower hexagonal faces, or by light being internally reflected within the crystal. Any refraction is cancelled out by the reflections: the crystals act only as a mirror, not a prism.
Your turn: how many halos can you identify in this photography from Falköping, Sweden in the winter of 2002-2003? Image credit: Nasko
So there it is: one photograph, eight types of ice halos, and a whole lot of distilled beauty. From the types of halos present, we know that the ice crystals were a mix of plates and columns, and that at least some of the columns were falling in a very specific orientation.
If you liked this, you may also enjoy puzzling over Bottlinger's Rings. To learn more, I recommend checking out the Atmospheric Optics, Cloud Appreciation Society, and Atmospheric Phenomena. Via NWS Amarillo's Facebook page.